Active sites in vanadia/titania catalysts for selective aerial oxidation of β-picoline to nicotinic acid

Vanadia/titania catalysts with varying vanadium content were prepared by impregnation using three different titania carrier materials of varying surface area. The structure of active vanadium species for β-picoline oxidation was investigated. Vanadium is mainly in the +5 oxidation state as revealed by electron paramagnetic resonance (EPR) and 51V magic-angle spinning nuclear magnetic resonance (51V MAS NMR) spectroscopy techniques. Diffuse reflectance UV–visible (DRUV–vis) spectroscopy and spectral deconvolution enabled identification of at least five different types of vanadium oxide species in these catalysts: monomeric tetrahedral VO3−4, polymeric distorted tetrahedral VO−3, square pyramidal V2O5, octahedral V2O2−6 and V4+ oxide species. While both VO3−4 and VO−3 species are active in β-picoline oxidation, the latter having a distorted tetrahedral geometry yielded the desired products—picolinaldehyde and nicotinic acid. High surface area, anatase structure for the support and dispersed, distorted tetrahedral vanadium oxide species are the key parameters determining the activity and selectivity of these oxidation catalysts.